1. Field of the Invention
The present invention relates to an optical transmission line suitably used for a large-capacity high-speed WDM optical transmission system, and an optical fiber suitably used for such an optical transmission line.
2. Related Background Art
An optical transmission system employing the WDM (Wavelength Division Multiplexing) scheme transmits a wavelength-multiplexed optical signal in the 1.55-xcexcm wavelength band through an optical fiber transmission network and enables large-capacity high-speed communication. This optical transmission system is constructed by an optical fiber transmission line as an optical signal transmission medium, an optical amplifier for amplifying a wavelength-multiplexed optical signal at once, and the like. Various researches and developments have been made to enable larger-capacity higher-speed WDM communication.
For an optical transmission line, reduction of dispersion and a dispersion slope is an important subject of study. More specifically, when an optical transmission line has dispersion in the wavelength band of an optical signal, the waveform of optical signal sent from the transmitting station deforms through the optical transmission line to cause reception degradation at the receiving station, because the optical signal has a certain bandwidth though the signal is monochromatic. Hence, dispersion in optical transmission line is preferably as small as possible in the signal wavelength band. For large-capacity communication, dispersion in optical transmission line is desirably small in a signal wavelength band as wide as possible. Hence, the dispersion slope in the optical transmission line is also preferably as small as possible.
Studies have been made to almost nullify both dispersion and a dispersion slope in an optical transmission line in the 1.55-xcexcm wavelength band. More specifically, a single-mode optical fiber having a zero dispersion wavelength in the 1.3-xcexcm wavelength band and positive dispersion and a positive dispersion slope at the wavelength of 1,550 nm and a dispersion compensating optical fiber having negative dispersion and a negative dispersion slope at the wavelength of 1,550 nm are connected and constructed as an optical transmission line, thereby almost nullifying both dispersion and a dispersion slope as a whole in the 1.55-xcexcm wavelength band for the optical transmission line. The present inventor, however, has found that the above-described optical transmission line formed by connecting an existing dispersion compensating optical fiber to a single-mode optical fiber is not always preferable for actual construction from the viewpoint of transmission loss and nonlinear optical phenomenon.
The present invention has been made to solve the above problem, and has as its object to provide a dispersion compensating optical fiber which has a small average transmission loss and can suppress a nonlinear optical phenomenon for an entire optical transmission line when connected to a single-mode optical fiber to form the optical transmission line, and an optical transmission line having such a dispersion compensating optical fiber.
According to the present invention, there is provided a dispersion compensating optical fiber having:
a dispersion value DDCF (unit: ps/nm/km) falling within a range of xe2x88x9282xe2x89xa6DDCFxe2x89xa6xe2x88x9229 at a wavelength of 1,550 nm;
a dispersion slope SDCF (unit: ps/nm2/km) falling within a range of 0.0023xc3x97DDCFxe2x89xa6SDCFxe2x89xa60.033+0.0015xc3x97DDCF at the wavelength of 1,550 nm; and
a transmission loss of not more than 0.5 dB/km at the wavelength of 1,550 nm.
When this dispersion compensating optical fiber is connected, at an appropriate length ratio, to a single-mode optical fiber having a zero dispersion wavelength in a 1.3-xcexcm band and positive dispersion at a wavelength of 1,550 nm, an optical transmission line which reduces both the transmission loss and nonlinear index can be formed.
Especially, the dispersion compensating optical fiber preferably has the dispersion value DDCF (unit: ps/nm/km) falling within a range of xe2x88x9282xe2x89xa6DDCFxe2x89xa6xe2x88x9236 at the wavelength of 1,550 nm. In this case, the nonlinearity of the dispersion compensating optical fiber is further reduced, and an optical transmission line with a smaller nonlinear index can be formed.
The dispersion compensating optical fiber according to the present invention preferably comprises a core region including an optical axis center and having a first refractive index, a first cladding region surrounding the core region and having a second refractive index lower than the first refractive index, and a second cladding region surrounding the first cladding region and having a third refractive index lower than the first refractive index and higher than the second refractive index. This easily realizes a dispersion compensating optical fiber having the dispersion value DDCF and dispersion slope SDCF which satisfy the above inequalities.
In this dispersion compensating optical fiber, a relative refractive index difference of the core region to the second cladding region preferably falls within the range of 1.3% to 1.7%, and a relative refractive index difference of the first cladding region to the second cladding region preferably falls within the range of xe2x88x920.5% to xe2x88x920.2%.
The dispersion compensating optical fiber according to the present invention preferably comprises a core region including an optical axis center and having a first refractive index, a first cladding region surrounding the core region and having a second refractive index lower than the first refractive index, a second cladding region surrounding the first cladding region and having a third refractive index lower than the first refractive index and higher than the second refractive index, and a third cladding region surrounding the second cladding region and having a fourth refractive index lower than the third refractive index and higher than the second refractive index. This easily realizes a dispersion compensating optical fiber having the dispersion value DDCF and dispersion slope SDCF which satisfy the above inequalities.
In this dispersion compensating optical fiber, a relative refractive index difference of the core region to the third cladding region preferably falls within the range of 1.3% to 1.7%, and a relative refractive index difference of the first cladding region to the third cladding region preferably falls within the range of xe2x88x920.5% to xe2x88x920.2%.
According to the present invention, there is also provided an optical transmission line formed by connecting:
a single-mode optical fiber having a zero dispersion wavelength in a 1.3-xcexcm wavelength band and a positive dispersion slope SSMF (unit: ps/nm2/km) at a wavelength of 1,550 nm and used for a cable line; and
a dispersion compensating optical fiber, when letting R be a DCF ratio representing a ratio of the length of the dispersion compensating optical fiber to the length of the entire cable line, having a dispersion value DDCF (unit: ps/nm/km) falling within a range of xe2x88x9282xe2x89xa6DDCFxe2x89xa6xe2x88x9229 at the wavelength of 1,550 nm, a dispersion slope SDCF (unit: ps/nm2/km) falling within a range of 0.0023xc3x97DDCFxe2x89xa6SDCFxe2x89xa6[0.03xe2x88x92(1xe2x88x92R)SSMF]/R (unit: ps/nm2/km) at the wavelength of 1,550 nm, and a transmission loss of not more than 0.5 dB/km at the wavelength of 1550 nm.
In this optical transmission line, both the transmission loss and nonlinear index are small. Especially, the DCF ratio R is preferably 0.2 to 0.4.
In the optical transmission line according to the present invention, the dispersion compensating optical fiber preferably has the dispersion value DDCF (unit: ps/nm/km) falling within a range of xe2x88x9282xe2x89xa6DDCFxe2x89xa6xe2x88x9236 at the wavelength of 1,550 nm. In this case, the nonlinearity of the dispersion compensating optical fiber is further reduced, and the nonlinear index of the optical transmission line itself becomes smaller.
In the optical transmission line according to the present invention, the single-mode optical fiber preferably has an effective sectional area Aeff of not less than 100 xcexcm2 at the wavelength of 1,550 nm. This suppresses the power density of propagation light and suppresses degradation in waveform due to the nonlinear effect.
In the optical transmission line according to the present invention, preferably, the single-mode optical fiber has a core region and a cladding region, and the core region is formed from silica which is not intentionally doped with an impurity, and transmission loss at the wavelength of 1,550 nm is not more than 0.18 dB/km. In this case, the loss in the entire optical transmission line can be reduced by making the Rayleigh scattering coefficient small. As a result, the power of incident light can be suppressed, and degradation in waveform due to the nonlinear effect can be suppressed.
In a dispersion compensating optical fiber according to the present invention, a minimum wavelength (to be referred to as a xe2x80x9cleading wavelengthxe2x80x9d hereinafter) at which an increase amount of an actual loss value with respect to a theoretical loss value is not less than 10 mdB/km in a use wavelength band and on a long wavelength side of the use wavelength band preferably falls within the range of 1,565 to 1,700 nm and, more preferably, 1,620 to 1,700 nm.
In this case, when this dispersion compensating optical fiber is connected, at an appropriate length ratio, to a single-mode optical fiber having a zero dispersion wavelength in a 1.3-xcexcm band and positive dispersion at a wavelength of 1,550 nm, an optical transmission line which reduces both the transmission loss and nonlinear index in the use wavelength band can be formed. If the use wavelength band is the C band (1,520 to 1,565 nm), the leading wavelength of the dispersion compensating optical fiber preferably falls within the range of 1,565 to 1,700 nm. If the use wavelength band includes not only the C band but also the L band (1,565 to 1,620 nm), the leading wavelength of the dispersion compensating optical fiber preferably falls within the range of 1,620 to 1,700 nm.
An optical transmission line according to the present invention is preferably formed by connecting a single-mode optical fiber having a zero dispersion wavelength in a 1.3-xcexcm wavelength band and positive dispersion at a wavelength of 1,550 nm to the above-described dispersion compensating optical fiber. In this optical transmission line, both the transmission loss and nonlinear index are sufficiently small.